Have impala 2000 3.4 engine, replace a bad pump that caused engine to overheat. From there on it has been a nightmare. Temp gauge goes Erratic. Then goes to were I had always seen it. Then goes erratic again.
I replaced again the water pump with the heavy duty, new radiator, new hoses, new thermostat, new temp sending unit, new fans, replaced fan relay. Fans do come in at slow speed when it reaches 220 degrees. (that is what my scanner states) temp goes to 220. Read with an infra red thermometer. It does go to 220 when fans come in at slow speed temp goes down stay there for 10 15 min then goes back up again.
Can the computer be reprogram to send signals to the fans before the 220.

Ensure that the system is purged of air. Were there any signs the Dex-Cool coagulated and blocked internal passages? Remove the spark plugs and check each cylinder's compression. This will help you determine if the head gasket, aluminum head, and valves have been damaged.

Replaced heads, The head are NEW. Compretion is fine better than when new. Everything that could be reolaced has been replaced, I have purged the system for air many tymes ( at least 6 or 7) Radiator will hold pressure of 25 lbs for ever. What in the world is wrong??

. Check/replace the Engine Coolant Temperature Switch, not sensor. This controls when the radiator fan comes on and goes off.

. Ensure the thermostat (OEM only) is installed properly and not sticking closed.

. Faulty radiator cap--doesn't maintain proper pressure or one or more valves are bad. Test cap relief pressure with a radiator cap tester (KD 3700 or Stant 12270). On non-expansion tank systems, which are most systems besides Ford's, there are two valves inside the cap that can fail. One allows hot coolant to flow into the overflow tank, the other allows coolant to flow from the overflow tank to the radiator, as the engine cools. If the coolant can't flow back into the engine, there won't be enough coolant and the engine can overheat. It's best to use a high quality, preferably OEM, cap.

I replaced radiator cap with a GM. Where is this temp switch.

Also, when the air condition is on and when the high pressure reading on the on the air condition manifold reaches 225 you can hear a (click) then both fans starts going for a brief moment, the driver side fan stop before than the pass side, the high pressure drop to 175 and temp goes back to normal. Car temp will fluctuate from 196 to 210. This cycle takes about 5 min. Temp reading are taken from my scanner and using an Infrared thermometer. The dash boad gauge reads this same temp. each line represent 10 degrees mid gauge is 210. If air condition is not on, the temp will go to 225, both fans come in at full blast but drop at 200 degrees.

I have had this car since nov 1999. For 11 yrs it has always ran with the temp gauge at 195. On highway or in stop and go traffic. Oil changes at 3000 miles, trans fluid replaced at the schedulled intervals. The Water pump failed last yr. I parked the car until this problem is fixed.

The ECT Switch (1-wire) is usually located in the side of the thermostat housing.

What I have next to the thermostat is the temp sensor switch. I can't find enything else there. The Sensor switch is what is there, and it has two wires these I have replaced twice.
Are these fans supposed to run when car is idling? Since there is no air flow through the radiator? They do if the air cond is on, and both fan come in and out .
When are these fans should come in? Temp goes up to 220 fans come in temp goes down to 200 this is with the air cond off. Then cycle starts again.
I have jet to find a satisfactory answer as to when and how they come in; Logit tells me that at least the one on the drivers side should be on, when idling or sitting still, there is no flow of air through radator.

Done that all relays are new, replaced them today. All fuses ck ok. Have a wiring diagram for the air cond , and relays, i will work on this tomorrow and fallow the trouble shut that was sent to me. The question still is; are these fans suppose to run while air is not on? If the air condition is on fans run intermitent now. On and off.

Well I believe the problem was solved. What I did was to raise the car about 20 inches from the front, used a hand vacuum pump and a funnel adde to the radiator. Filled with antyfreeze. Statrted pumping removed almust 1/4 gal when a big buble of air came out refilled the funnel and did the same again this time only a few bubles came out. Did this until only liquid came out ( for about 10 min). Tested the car. WOW!! Now the temp gauge is where it was when new .
Gauge does fluctuate from 194 to 198 when idlyng . With air cond on does not move. Same with the heatter stay at 194/195.
Thanks for all the help.
Jose

Nice job!